The problem of deep drilling is sufficiently known in the metal cutting industry. Manufacturers of numerical control systems offer finished programs. Upon reaching the preprogrammed depth, the drill is retracted, according to the program, in order to remove the chips. Afterwards the drill returns to the already reached boring depth and continues the drilling operation. This procedure is possible when applied to metal cutting operations, because the machining parameters such as speed and rate of advance can be maintained.
The situation is different for ultrasonic erosion (see "Keramikbearbeitung", Carl Hauser Verlag, Munich 1989, pages 423 to 443). In this case the force feedback control guides the abrasion process. That is, the issue is not to retract at given depths but rather to adapt as optimally as possible to the actual process-related possibilities. The retraction of the sonotrodes should be possible at any time and at any arbitrary boring or sink depth, in order to increase the removal capacity. By unloading and lifting the sonotrodes, sharp new abrasive grains may be applied to the working area. This situation is especially important when no vacuum can be employed to activate the replacement of the abrasive grains.
Numerically controlled retracting cycles are not very suitable for this purpose. They are tied to position, and cannot be employed arbitrarily in time. They are complicated, because the already reached boring depth has to be stored. After one retracting cycle the tool returns automatically to the already bored depth. In so doing, an overload or shocks can occur, because during the retracting operation the borehole or recessed surface is covered with an abrasive compound.